(i) Field of the Invention
The present invention relates to an impulse high voltage generator, and more particularly to a high voltage generator which can vary a peak value of an impulse high voltage and a frequency of generating the voltage simply and inexpensively with high efficiency.
(ii) Related Art Statement
In a conventional impulse high voltage generator, as shown in FIG. 16, an output voltage VTr of a DC high voltage power source 1 is impressed aross an energy storing capacitive element 5 such as a high voltage condenser and a high voltage coaxial cable through conductors 2, 3 and a current-limiting resistor 4 to charge the capacitive element. The electrical charge stored in the capacitive element is discharged to a discharging resistor 7 through conductors 8, 9 by means of a high-speed switching device 6 to impress an impulse high voltage to a load 12 through output terminals 10 and 11. The high-speed switching device 6 mainly utilizes a spark gap for use in the impulse high voltage generator, and the spark gap includes electrodes 13 and 14 spaced from each other properly and enclosed with insulating gas of a proper type and having proper pressure. The high-speed switching device 6 may be a hydrogen thyratron or a high-speed thyristor.
When the high-speed switching device 6 is the spark gap, the high-speed switching divice 6 has a distance between the electrodes adjusted so that the spark voltage Vs is a value lower than the output voltage VTr of the DC high voltage source 1. When the voltage Vc across the capacitive element 5 reaches the spark voltage Vs in the course of the charging operation, spark is produced across the electrodes 13 and 14 to short-circuit between both the electrodes immediately and the electrical charge stored in the capacitive element 5 is discharged through the conductors 8 and 9, the spark gap 15 and the discharging resistor 7 to generate a sharply rising impulse high voltage across the output terminal 10 and 11.
The above-mentioned spark gap is of self-blasting type. Alternatively, in order to start the spark gap, an externally controlled spark gap has been used in which the voltage Vc is established to be Vs&gt;Vc and a pulse voltage is superposed on the voltage Vc to trigger the spark gap, or in which a third trigger electrode is provided between the electrodes 13 and 14 and a trigger pulse voltage is applied to the third trigger electrode.
The conventional well-known impulse high voltage generator shown in FIG. 16 has large drawbacks described below irrespective of the self-blasting type and the externally controlled type.
(1) When the energy storing capacitive element (hereinafter referred to as a capacitor) 5 is charged, the output voltage VTr of the DC high voltage source 1 is impressed through a series circuit of the current-limiting resistor 4 and the capacitor 5 to the condenser 5. The current-limiting resistor 4 dissipates energy equal to energy of CVc.sup.2 /2 supplied to the capacitor 5 having a capacitance C as Joule loss, resulting in remarkable power loss.
(2) Since the current-limiting resistor 4 suppresses a current flowing from the DC high voltage source 1 when the high-speed switching device 6 is turned on, the resistor 4 has a large resistance value such as 0.5-10 megohms. This is because the spark of the high-speed switching device 6 is shifted to a continuous arc discharge and the current of I=VTr/(Rc+Rm) flows continuously if the current from the DC high voltage source 1 is not suppressed when the switching device 6 is turned on. In the equation of the current, VTr is the output voltage of the DC high voltage source 1, Rc is a resistance value of the current-limiting resistor 4, and Rm is a resistance value of the discharging resistor. The resistance value Rc of the current-limiting resistor 4 is largely related to a time constant .tau.=Rc.multidot.C for charging the capacitor 5. If the resistance value Rc is increased excessively, the time constant becomes large and the impulse high voltage can not be generated frequently to limit the frequency of generating the impulse high voltage. On the other hand, if the resistance value Rc is decreased, the above-mentioned continuous current becomes large. Accordingly, the resistance value Rc is not decreased too small. In any event, the current-limiting resistor 4 requires large resistance value of 0.5-10 megaohms, large power capacity, and cooling facilities because of the reason described in the above item (1).
In order to solve the problems described in the above items (1) and (2), Japanese Patent Application No. 56-144399 entitled "Vary Short Pulse High Voltage Generator" filed in 1981 in the name of Senichi Masuda discloses that the current-limiting resistor 4 of FIG. 16 is removed as shown in FIG. 17 and an AC high voltage power source 21 is used in place of the DC high voltage power source 1. A rectifier 22 is connected to an end of an output of the source 21 and a capacitor 23 is charged through the rectifier 22 from a zero voltage to an AC output peak voltage Vm, so that power loss for charging is suppressed to extremely small value in principle. The high-speed switching device 24 uses the externally controlled high-speed synchronizing switch which switches at a high speed at a predtermined time in synchronism with the AC voltage and the synchronizing switch is turned on during a half cycle of the AC high voltage when the polarity of the AC high voltage is inverted so that the rectifier 22 prevents the capacitor 23 from being discharged. Thus, the electrical charge stored in the capacitor 23 is discharged through a discharging resistor 25. However, this apparatus of the power frequency synchronizing type has also drawbacks described below.
(1) The half-wave rectification type DC high voltage power sorce is used for a power source for charging the capacitor actually, and since the voltage at the secondary side of the high voltage transformer is half-wave rectified, the primary AC current is unsymmetrical and the distorted excitation of the transformer tends to be produced. Accordingly, the countermeasures such as increasing the capacity of the transformer and use of a center-tapped transformer (FIG. 18) are required.
(2) Since the above apparatus operates in synchronism with the frequency of the power source completely and the capacitor 23 is charged by the half-wave of the AC high voltage power source 21, the occurrence frequency of the impulse high voltage is fixed to 50 or 60 times per second (equal to the power frequency). Accordingly, the occurrence frequency can not be changed and the frequency exceeding 50 or 60 times per second can not be realized.